Data on total and spectral solar irradiance: comments
C. Fröhlich Physikalisch-Meteorologisches Observatorium, World Radiation Center, Davos, Postfach 173, 7260 Davos Dorf, Switzerland. Received 27 June 1983. 0003-6935/83/243928-01$01.00/0. © 1983 Optical Society of America.
This Letter is in response to a paper by Mecherikunnel et al.1 In recent years our knowledge of the solar constant has significantly increased, and modern values are, although referenced, completely ignored by the authors of the above article. The best value today is between 1366 and 1368 W m - 2
(Refs. 2 and 3) and not 1353 W m - 2 . The latter is based on the mean of measurements from airplanes, which we know are not very reliable,4 mainly due to the uncertainty of the atmospheric and window transmissions. Furthermore, the radiometric scale used is off by more than 1%.
As to the spectral irradiance, the ignorance of the authors is even more striking, and it is difficult to understand why this article appeared in a reputable journal such as Applied Optics. Since the data were published by Thekaekara5 about ten years ago, they are questioned for several reasons: inconsistency of the different data sets used to produce the final results; the correction methods adopted; calibration standards and procedures used.6 Even at that time it was obvious, by comparison with the Labs and Neckel data, that systematic errors must have influenced the Thekaekara spectrum. Some can be explained by the use of NBS irradiance standards, the calibration of which was at that time off by a few percent.7
Due to the fact that Labs and Neckel8 measured the radiance at the center of the solar disk and used published limb darkening data to deduce the irradiance at 1 astronomical unit, the reliability of data could obviously be questioned. However, in the meantime this drawback of the data set has been eliminated (Neckel and Labs9), and it is demonstrated that the differences between the old and new spectra are small. Even before the publication of Ref. 8 and after application of the corrections in Ref. 9, independent determinations by Shaw and Fröhlich10 showed good agreement with the data of Ref. 8. Fröhlich and Wehrli11 also find evidence that the revised Neckel and Labs spectrum is an improvement over the former version and the absolute uncertainty is now 1-2% (Ref. 2) (Fig. 1). This latter statement is supported further by the most recent limb darkening data gathered by Neckel and Labs12
at Kitt Peak with the same spectrometer as used at Jung-fraujoch in the 1960s for determination of the center radiance. Another indication of the quality of this spectrum is that the integral over the Neckel and Labs spectrum yields a value in very close agreement (within 0.2%) with the most modern determinations of the solar constant.2,3
3928 APPLIED OPTICS / Vol. 22, No. 24 / 15 December 1983
Fig. 1. Comparison of extraterrestrial solar spectral irradiance determined by sun photometers10,11 and by Neckel and Labs.9 The circles and square are determinations from Mauna Loa,10 the asterisk from a stratospheric balloon.11 The circles are calibrated against an NBS irradiance standard lamp, the square and the asterisk with the dye laser method at PMOD. The error bars indicate estimated ab
solute accuracy.
From the above one can conclude that at present the uncertainty of the solar spectral irradiance in the 330-1000-nm range is of the order of 1-2% and not, as stated by Mecherikunnel et al., 10-15%. Thus the NASA/ASTM spectrum is incorrect over wide parts of the spectrum and should no longer be used as a reference or for design purposes.
As a consequence, the published results for the irradiance at different airmasses are wrong by as much as 10-20% at some wavelengths due to the wrong spectrum and the wrong integral value. They are, therefore, not very useful. Furthermore, the conclusion of the paper that "as airmass increases, the amount of energy in the infrared relative to the total increases and that the energy in the UV and visible decreases" is well known; indeed the sun is red at low elevations.
The author gratefully acknowledges J. Geist, NBS, Washington, D.C. for suggesting this Letter and for helpful comments on an earlier version.
References 1. A. T. Mecherikunnel, J. A. Gatlin, and J. C. Richmond, Appl. Opt.
22, 1354 (1983). 2. C. Frohlich and R. W. Brusa, Sol. Phys. 74, 209 (1981). 3. R. C. Willson, Sol. Phys. 74, 217 (1981). 4. C. Fröhlich, in Solar Output and Its Variation, O. R. White, Ed.
(Colorado Associated U.P., Boulder, 1977). 5. M. P. Thekaekara, Sol. Energy 14, 109 (1973). 6. E. Flowers et al., in Solar Energy Data Workshop, C. Turner, Ed.
(U.S. GPO, Washington, D.C, 1974), p. 208. 7. R. D. Saunders and J. B. Shumaker, NBS Tech. Note 594-13
(1977). 8. D. Labs and H. Neckel, Sol. Phys. 15, 79 (1970). 9. H. Neckel and D. Labs, Sol. Phys. 74, 231, (1981).
10. G. E. Shaw and C. Fröhlich, in Solar-Terrestrial Influences on Weather and Climate, B. McCormac and B. Seliga, Eds. (Reidel, Dordrecht, Holland, 1979), p. 69.
11. C. Frohlich and C. Wehrli, in Proceedings, Third Scientific Assembly of IAMAP, Hamburg, 1981; The Symposium on the Solar Constant and the Spectral Distribution of Solar Irradiance (Boulder, 1982).
12. H. Neckel, H. Sternwarte, and D. Labs, Landessternwarte Heidelberg; private communication (1983).
